Roosting Behavior of Premigratory Dunlins (Calidris alpina)

Abstract

-We studied roosting behavior of Dunlins (Calidris alpina) during late summer along the coast of the Yukon-Kuskokwim Delta, Alaska, in relation to tidal cycle, time of day, time of season, and occurrence of predators. Within Angyoyaravak Bay, peak populations of 70,000-100,000 Dunlins occur each year. The major diurnal roost sites were adjacent to intertidal feeding areas, provided an unobstructed view of predators, and were close to shallow waters used for bathing. At one site studied intensively, roosting flocks formed at high water consistently during the day but rarely at night. On about 75% of the days, Dunlins also came to the roost at dawn and dusk when the tide was low. The size of the roosting flock, the length of time birds spent at the roost site, and behavior at the roost site were highly variable throughout the season and significantly affected by both tide level and time of day. Roosting behavior changed significantly between early and late August, as Dunlins underwent heavy wing and body molt, and began premigratory fattening. The reaction of Dunlins to potential predators, the formation of roosting flocks in response to light cues, and seasonal changes in social behavior at the roost site suggested that communal roosting behavior may be related not only to the risk of predation but also to behavior during migration. Received 9 July 1990, accepted 5 August 1991. FEW investigators have examined the roosting behavior of shorebirds in light of either the physical factors that govern it or the benefits that might accrue (for review, see Myers 1984, Hockey 1985). Postbreeding Dunlins (Calidris alpina) roost in large flocks and feed on expansive intertidal mudflats of the Yukon-Kuskokwim Delta, Alaska, from July until early October (Holmes 1971, Gill and Handel 1990). At this time they are in transition from the breeding period, during which they defend dispersed nesting and feeding territories (Holmes 1966a, b, 1970, Pitelka et al. 1974), to the wintering period, during which they are highly social, and most commonly feed and roost in flocks along the coast (Storer 1951, Page 1974, Page and Whitacre 1975, O'Connor 1981, Brennan et al. 1985, Ruiz et al. 1989). While on the Yukon-Kuskokwim Delta, Dunlins complete prebasic molt (Holmes 1971) and prepare for a transoceanic migration to wintering areas in southeastern Asia and California (Gill and Handel 1990, in prep.). We expected that their feeding and roosting schedule would be dictated primarily by physical factors, such as the tidal cycle, that affect the profitability of foraging (Heppleston 1971, Prater 1972, Burger et al. 1977, Hartwick and Blaylock 1979, Kelly and Cogswell 1979, O'Connor 1981, Zwarts et al. 1990). We examined the patterns of formation and dispersal of roosting, premigratory Dunlin flocks to determine what factors might influence roosting behavior during this important part of their annual cycle.